Valve assemblies for the purpose described above usually comprise a deformable plug having a cylindrical body which is situated in a housing providing a passageway for fluid. The plug is seated within a valve seat in the housing between two backing plates forming part of the assembly. The plug outer diameter is such that when unstressed there is provided between the plug and the valve seat an annular clearance for the flow of fluid. In use one backing plate is fixed to the housing and other plate can be moved towards the fixed plate by some form of axial stressing means, such as a movable cylinder, so that the plug is stressed between the plates and radially expands to restrict the annular clearance and thereby fluid flow.
The cylinder is hydraulically linked to a diaphragm motor assembly which serves to provide fine adjustment of the pressure of the hydraulic fluid and thereby of the cylinder movement. A typical conventional valve assembly and fluid flow regulator is described in some detail in UK patent specification No. 1035427.
One problem with conventional valve assemblies is that during use the deformable plug is subject to severe erosion. Consequently the plugs must be replaced at relatively frequent intervals and as these plugs are relatively expensive this is a considerable drain on operating costs.